Metal production has evolved over time from the simple heating of ores to electrolytic processes such as those used in aluminum smelting. In the electrolytic aluminum smelting process, electrical current is applied to aluminum oxide through carbon anodes and cathodes. During the smelting process the carbon anodes, and to a lesser extent the cathodes, are consumed. In this application the term “anode” is used for simplicity, and not as a limitation.
Because electrolytic smelting relies upon the passage of an electric current through the anodes, impurities or defects which increase electrical resistance result in an undesirable increase in electricity consumption. Furthermore, impurities in the anodes can contaminate the melt, resulting in poor quality metals. A high-quality carbon anode typically contains less than 8% of volatiles and is properly baked by uniform heating to a specified temperature range. Improperly baked anodes may have higher than desired electrical resistance as well as physical characteristics such as hardness which are substandard.
In typical aluminum smelting, every two kilograms (kgs) of smelted aluminum consumes approximately one kg of carbon from a carbon anode. Given that worldwide smelting capacity of aluminum exceeds 42 million metric tons, there is a significant and ongoing demand for high-quality carbon products.
Currently, this demand is satisfied using several furnace technologies such as closed top furnaces (“CTF”) and more commonly open top furnaces (“OTF”), which are also known as “ring furnaces.” Due to operational difficulties including a risk of explosion, CTF's have met with disfavor in the industry and are no longer considered viable. Thus, the majority of current carbon anode production takes place in OTFs.
An OTF is constructed by building fixed pits which are surrounded by flues and headwalls. Refractory ducts at each end of the furnace known as “crossovers” provide a means of reversing gas flow to help create a continuous ring. Unbaked (or “green”) carbon anodes are placed into these pits. To prevent slumping and air burning of the anodes, petroleum coke is packed around and on top of the anodes. A flammable material is fed into the flues and combusted, with the exhaust gas drawn off. The pit walls are permeable, and a slight negative pressure in the flues draws the volatile gasses from the carbon anodes into the flues where they may be combusted. Failure to combust these gasses may result in an explosion hazard.
In the conventional OTF, there may be dozens or hundreds of pits arranged in a grid. These pits and the surrounding flues, headwalls, and crossovers are built from thousands of tons of masonry which require large amounts of time and expensive skilled labor. Further complicating design and construction of an OTF facility is the need to accommodate moving large masses such as anodes, coke, and equipment with large, heavy, multi-purpose overhead cranes during production, resulting in expensive buildings with high ceilings and clear building spans.
In operation, each group of pits and flues between a pair of headwalls (a section) may be undergoing a different step in the production process. For example, one section may be heating up, while an adjacent section is performing a fired soak. An adjacent section may be cooling, while the next section is being unloaded. Meanwhile an empty section may be undergoing cleaning and maintenance for the next round of firing.
During carbon anode production in an OTF large “frames” are moved between the sections. Because the sections are fixed and may be at different phases of the production process as described above, a frame is lifted over other frames and placed in the appropriate section. This is one of the reasons why an OTF facility must have a high ceiling with clear spans. There are several different kinds of frames which must be moved, including fire frames, cooling frames, exhaust gas frames, instrument bridges, etc.
Movement of a fire frame is referred to as a “fire move.” Because of the noxious fume and dust hazards involved in this highly dangerous environment, non-essential plant personnel are often evacuated during a fire move. Further complicating the production process in an OTF is the requirement to perform a fire reversal. Under the high temperatures experienced during operation, over time the headwalls may begin to lean or shift as the flues move in the direction of the fire. To even out this leaning, the direction of combustion gasses in flues may be changed. This change is known as a “fire reversal.” However, given the complicated interconnection of headwalls and flues found in the OTF, performing a fire reversal is difficult. A fire reversal affects every section in an OTF, even though some sections may not require this adjustment. Because each flue reacts differently in operation, this may result in a fire reversal being made in sections where it is not required. In other words, the control over fire reversal in an OTF is too coarse.
The fixed arrangement of the OTF facility makes maintenance to the pits, flues, headwalls, and other equipment difficult. A pit, flue, or headwall in a section in need of repair may be immediately adjacent to a hot section, thus it cannot be repaired until the firing equipment has moved past and this area has been allowed to cool. Because of the operational shortcomings of the OTF, these sections often do not cool adequately. Quite simply, it is difficult and dangerous to make extensive repairs when the section being repaired is hot enough to broil a worker. As a result necessary repairs may be done without due care, or not done at all.
Additionally, the OTF design wastes a tremendous amount of energy. The complicated system of pits, flues, headwalls, crossovers, etc., results in many avenues for heat to escape, cold air to enter, excess heavy refractory that absorb heat, etc. Although energy can be recovered during the OTF process and used downstream, such recovery is severely compromised by these losses. As a result, large quantities of energy, typically delivered by the combustion of fossil fuels, are used. This results in significant outputs of carbon dioxide, as well as volatiles which emanate from the anodes during baking.
The OTF design also results in significant quantities of coke dust and exhaust gas. Loading and unloading of the carbon anodes generates coke dust. For example, loading a pit involves dumping coke into a pit, releasing clouds of coke dust. Unloading similarly stirs up coke dust. Movement of the carbon anodes covered with coke dust through the facility spreads coke dust even farther.
Exhaust gas results from the combustion of volatiles outgassed by the carbon anodes and supplemental fuel. To comply with environmental regulations, in many countries this exhaust gas must be scrubbed, or chemically processed, to remove pollutants. The larger the volume of exhaust air to be treated, the more extensive and expensive the scrubbers must be. The arrangement of pits, flues, and headwalls in an OTF results in a significant amount of air being drawn into the system, resulting in larger volumes of exhaust gas than necessary being produced. Furthermore, failure to combust outgassed volatiles may result in a further explosion hazard, such as occurs in CTFs.
Finally, the refractory and other equipment in an OTF is under constant attack. Constant thermal cycling expands and contracts materials, resulting in cracking. Volatiles outgassed from the carbon chemically attack refractory and other equipment. Movement of frames and the process of loading and unloading anodes results in mechanical damage when equipment hits pit walls, flues, and so forth. Given the combination of harsh environment and difficulty in performing ongoing maintenance, the OTF facility quickly falls into a poor operational state. Productivity drops, as does the quality of the anodes produced. To remedy the problem, the entire facility, or at least a major portion, must be shutdown, cooled, demolished, the rubble removed, and rebuilt. This is dangerous, inconvenient, time consuming, wasteful, and expensive work. This work incurs loss of production, and often includes the need for major repair to support equipment such as conveyors and cranes.
Thus, there is a significant need for a process and apparatus to produce high-quality carbon products without the significant drawbacks currently found in OTFs.